Can I recycle a mech from my hand to activate Rumble Hotheaded?

No, you cannot recycle a Mech from your hand to activate the ability of Rumble, Hotheaded.

According to RiftJudge FAQ #3053, the term "unit" without further specification refers only to units on the board (base or battlefield). Units in your hand are in a non-board zone and are not considered "units" for the purpose of Rumble's ability.

Additionally, Rumble's ability specifically requires you to recycle a unit to play a Mech from your trash, not from your hand.